Because you are actually free to do what you want with it, instead of free to do what someone else wants you to do with it.
It's kind of like arguing that a country where anyone can steal from anyone else with impunity is more free. Not when you consider the rights of the person being stolen from.
Also, even if a project is copylefted, people can still just do... exactly what they did here. Which, while different in the weak sense of “avoiding copyright” or maybe “avoiding patents”, in the context of systems code like this almost always results in the same code on both sides anyway. If the choice is between either giving the proprietary developers your code to use, or making them re-implement exactly what you wrote without your copy for reference—with no option for “they don’t implement it at all”—then exactly what is the point of choosing the latter over the former?
We aren't talking about non-free, we're talking about less-free.
> Just because some forks of a project aren’t free, doesn’t mean that the project itself isn’t free
No. The fact that there can be forks of the project that aren't open is what means that the project itself is less free than a project where all forks must be open.
> Especially if the project, like much of the e.g. MIT-licensed code in the world, is “done”
I don't consider this relevant to the argument at hand.
> then exactly what is the point of choosing the latter over the former?
Are you asking me what the point is of making something you don't want people to do hard for them vs making it easy?
Additionally, "theft" as you put it, in this case, doesn't affect the original property owner.
Proclamations of rights aren't really useful if they don't have means for enforcing those rights are not taken away.